The photoresponse and thermoelectric properties in chemically modified graphene

Abstract

The novel single-atom-thick graphene has been intensively studied since its emergence owing to its intriguing physical properties such as, high electron mobility, gapless Dirac-cone band structure and room temperature ballistic conduction. The potential applications of graphene, from field-effect transistors, photodetectors, biological sensors to a heat dissipation medium, have been extensively investigated. Photogating effect has been utilised to realise high photoresponsivity in semiconductor-graphene hybrid photodetectors. Despite their great performance, the spectral response was inevitably limited by the bandgap of the semiconductor involved in the hybrid photodetector, partially compromising the merit of broadband absorption of graphene. This limitation can be circumvented by exploiting the electron-accepting ability of an electrochemical half-reaction. In the graphene phototransistor based on the electrochemical half-reaction, the effective reversible electron reservoir can store the hot electrons excited from graphene, which enlarges the sub-bandgap photoresponse in silver chloride (AgCl)-graphene photodetector. A photoconductive gain of ~ 3  109 electrons per photon in the photodetector is realized by the aid of the long lifetime of photoexcited carriers in chemically reversible redox couple AgCl/Ag0, which enables a pronounced visible light responsivity, beyond the band-edge absorption of AgCl (382 nm). This work not only provides an alternative to achieve electrically tunable, sub-bandgap photo detection in semiconductor-graphene heterostructures but also opens up a new avenue for utilising an electrochemical half reaction in other 2D systems and optoelectronic devices. Besides the photodetector application of graphene, the research on its thermoelectric properties have been less focused. Nowadays, the continuous scaling down in silicon-based transistor industry gives rise to the challenge of power dissipation. Therefore, a material with high thermal conductivity and good compatibility with other nanometer scale circuit components is highly desired. For thermoelectric devices, a low thermal conductivity material is instead preferred to achieve a high heat to electricity conversion efficiency. To study the thermoelectric properties in graphene at the nanoscale, soft and hard templates were utilized to pattern graphene into nanomesh structures. In addition, chemical modifications were investigated for its influence on the thermal properties of graphene. The Seebeck coefficient, electrical conductivity and thermal conductivity were explored. Moreover, an electric field by back-gate voltage was utilized to adjust the Fermi level in graphene and subsequently its Seebeck coefficient, which is difficult for bulk thermoelectric materials as a result of electrostatic screening. These results provide a basis to evaluate graphene as a thermal management medium in nanoscale circuits or as a candidate for thermoelectric application.